Phase 4: applications + SASL (sasldb2) + sender_login_maps

Adds application accounts bound to domains: a SASL login/password in
sasldb2, a per-application address mode (wildcard @domain or an explicit
list), and matching smtpd_sender_login_maps bindings — with create,
list, edit-mode, delete and password regeneration (spec 4.1, 5.1,
7.2.5-9). Generated passwords are shown exactly once and never stored in
plaintext (7.6.1).

- internal/store/applications.go: transactional CRUD; globally unique
  login; ListBindings (address->login) as the map source; logins-by-
  domain for pre-cascade SASL cleanup.
- internal/app: saslpasswd2 wrapper (password via stdin, login as a
  whitelisted argv element, no shell — 7.6.3); strong base64url password;
  address validation that enforces domain ownership before any config
  write (7.6.2); service orchestrating store + sasldb2 + map with full
  rollback on partial failure.
- internal/postfix: sender_login_maps regenerated as a pure function of
  the registry (many-to-one logins merged per address), atomic write,
  injection backstop (7.6.4).
- Postfix reload, corrected: `postfix start-fg` forks a separate master,
  so signalling the supervised process never reaches it. Reload now runs
  the canonical `postfix reload` via a one-shot supervisord program the
  unprivileged panel triggers over the group control socket. Verified in
  mail.log.
- domain.Service.Delete purges the domain's SASL accounts, then cascades,
  then rebuilds the sender map and reloads; manual reload now covers both
  OpenDKIM and Postfix.
- web: application management in the domain page, one-time credential
  shown inline; postfix joins the selfpost group and entrypoint normalises
  /data/sasl and /data/postfix (setgid, group-readable) with self-heal.

Verified on the dev server: gofmt/vet/test green, image builds, and a
container e2e covers the full application lifecycle, domain-delete
cascade, restart persistence, and a real postfix reload.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

build/Dockerfile

@@ -58,15 +58,17 @@ RUN postconf -e "maillog_file=/var/log/mail.log"
 # Unprivileged user for the panel process (spec 7.6.8).
 RUN useradd --system --no-create-home --shell /usr/sbin/nologin panel
 
-# Shared group bridging the two unprivileged services (spec 6): the panel
-# generates per-domain DKIM keys and rewrites the OpenDKIM tables, while
-# OpenDKIM (a different user) must read them. Membership in this group — plus
-# setgid dirs under /data/opendkim (set up in entrypoint.sh) — lets OpenDKIM
-# read the panel-owned keys, and lets the panel reach the supervisor control
-# socket to signal an OpenDKIM reload without any process running as root.
+# Shared group bridging the unprivileged services (spec 5.1, 6): the panel
+# generates per-domain DKIM keys, application SASL accounts (sasldb2) and the
+# Postfix sender map, while OpenDKIM and Postfix (different users) must read
+# them. Membership in this group — plus setgid dirs under /data (set up in
+# entrypoint.sh) — lets OpenDKIM read the panel-owned keys and lets Postfix read
+# the sasldb2/sender map, and lets the panel reach the supervisor control socket
+# to signal OpenDKIM/Postfix reloads without any process running as root.
 RUN groupadd --system selfpost \
  && usermod -aG selfpost panel \
- && usermod -aG selfpost opendkim
+ && usermod -aG selfpost opendkim \
+ && usermod -aG selfpost postfix
 
 # Runtime directories: milter sockets and the consolidated persistent root.
 RUN mkdir -p /run/opendkim /run/selfpost /data \